0to1650
I recommend humming under water. You slowly release bubbles, so you are almost (not completely) out of breath before you inhale.
I see beginners try to exhale and inhale while head it out of the water but you can maybe do a couple of 50 yard laps then feel exhausted. Just give it time.
Sigh, took a spill. Road rash city. So I need to stay out of the pool and lakes for two weeks so this will kill my swimming for my next tri (ocean swim).